What Caused the Hole in That Southwest Plane?

ABC's Lisa Stark tonight with an update on where the investigation stands: ABC News has learned that investigators can see no obvious signs of corrosion or metal weakness in the area of the 14 by 17 inch hole. Sources say corrosion now appears unlikely as a cause, but metal fatigue, tiny cracks could be a culprit — investigators will look at the metal under a microscope. The Southwest 737 is 15 years old, with 42,569 takeoffs and landings — and it appears the plane was NOT yet part of an aging aircraft program..that would have required intensive inspections for cracks and weaknesses.